The invention relates to a stabilized power supply unit having a current limiting function for maintaining at a constant level the output voltage supplied to a load if the output current to the load has changed, and restricting excessive output current to the load.
A stabilized power supply unit having a current limiting function is widely used in a series regulator serving as a convenient power supply and a constant voltage charging apparatus for charging a battery.
FIG. 4 shows a circuit structure of a series regulator having a conventional current limiting function.
The series regulator shown in FIG. 4 is composed of a voltage control circuit 10, an output circuit 20, and a current limiting circuit 30, integrated on an IC chip.
The voltage control circuit 10 is provided with a differential amplifier Amp and voltage dividing resistors R11 and R12. The differential amplifier Amp is provided at one input thereof (inverting input) with a reference voltage Vref for setting an output voltage, and at another input thereof (non-inverting input) with an output feedback voltage Vfb obtained by dividing the output voltage by the voltage dividing resistors R11 and R12. The difference between the two inputs is amplified by the differential amplifier Amp, and outputted from the voltage control circuit 10 as a control voltage Vc. The differential amplifier Amp is supplied with a constant current from a constant current source 11.
The output circuit 20 has an output transistor Q21 consisting of a p-type MOS transistor (hereinafter referred to as p-type transistor) connected between a power source potential Vdd and the output terminal Po of the power supply unit. The control voltage Vc is applied to the gate of the output transistor Q21. Connected to the output terminal Po is a load Lo and a condenser Co for stabilizing the output to the load.
The current-limiting circuit 30 includes a p-type current detection transistor Q31 and a detection resistor R31 connected in series in the order mentioned, between the power source potential Vdd and the ground. The current limiting circuit 30 is also provided with an n-type MOS transistor (hereinafter referred to as n-type transistor) Q32 having a gate impressed with the voltage drop across the resistor R31. Constant voltage control function of the voltage control circuit 10 is regulated by the operating condition of the n-type transistor Q32.
The detection transistor Q31 is formed together with the output transistor Q21 on the same IC chip with a predetermined ratio less than 1 in size as compared with the output transistor Q21. The gate of the n-type transistor Q31 is impressed with the same control voltage Vc as the gate voltage of the output transistor Q21. As a consequence, a detection current Ioxe2x80x2 which is practically proportional (e.g. 1/100) to the output current Io flowing through the output transistor Q21 flows through the n-type transistor Q31. The voltage drop across the detection resistor R31 by the detection current Ioxe2x80x2 determines the operating condition of the n-type transistor Q32. The threshold voltage of the n-type transistor Q32 is set to the voltage that corresponds to the output current (i.e. load current) Io being a preset over-current protection level Is0. The threshold voltage is determined by the ratio of the output current Io and the detection current Ioxe2x80x2, the resistance of the detection resistor R31, and properties of the n-type transistor Q32.
Operation of the conventional series regulator will be discussed with reference to FIG. 5 showing a characteristic relationship between the output voltage Vo and the output current Io of the regulator. Under normal condition in which the output current Io is below the limit of over-current, the voltage control circuit 10 outputs a control voltage Vc so as to equalize the output feedback voltage Vfb with the reference voltage Vref. This control voltage Vc is applied to the gate of the output transistor Q21 of the output circuit 20 to bring the output voltage Vo to a predetermined set voltage Vs. In this way, the constant voltage control of the regulator can be maintained stable at all times regardless of the magnitude of output current Io, unless the output current Io reaches the over-current protection level Is0.
Under such stable condition, the voltage drop by the detection resistor R31 due to the detection current Ioxe2x80x2 does not reach the threshold voltage of the n-type transistor Q32. Hence, nothing affects the constant voltage control function of the regulator.
However, as the output current Io reaches the preset over-current protection level Is0, the voltage drop across the detection resistor R31 reaches the operating threshold voltage of the n-type transistor Q32. Thus, the n-type transistor Q32 becomes operative as the output current Io exceeds the over-current protection level Is0. In the voltage control circuit 10, current limiting operation is prioritized, so that the output voltage falls quickly, almost vertically. In this sense, this over-current protection function has a drop-type characteristic. The current level Is1 at which the output voltage fully drops down to Vo is slightly higher (by the amount of xcex1) than the preset over-current protection level Is0, in accordance with the gain (control gain) of the current limiting regulator. The region above the level Is0 is an over-current region.
In this way, under normal condition the output voltage Vo is controlled to be at a preset voltage Vs. However, if the output current exceeds a predetermined level (over-current protection level Is0), the output current Io is automatically limited.
However, the drain voltage of the output transistor Q21 will be fixed to a predetermined set voltage Vs even if the output current changes, since the drain voltage is controlled to maintain a constant voltage at all times. On the other hand, the drain voltage of the detection transistor Q31 varies with the detection current Ioxe2x80x2, since the drain voltage depends on the product of the detection current Ioxe2x80x2 and the resistance of the detection resistance R31. Thus, even if the gate voltages Vc of the output transistor Q21 and of the detection transistor Q31 are the same, and hence so are the gate-source voltages Vgs, their drain-source voltage Vds can differ.
If the drain-source voltage Vds of the detection transistor Q31 changes, the detection current Ioxe2x80x2 will be changed according to the inclination of the static drain voltage versus drain current characteristic of the detection transistor Q31, if the gate of the transistor Q31 is impressed with the same gate voltage Vc to the output transistor Q21.
Therefore, the detection current Ioxe2x80x2 is not exactly proportional to the output current Io. Hence, the output current cannot be limited accurately to an over-current protection level Is0, to which the current should be limited. For this reason, it is often the case that the over-current protection level Is0 is set with some margin, or the output transistor Q21 is provided with a large over-current tolerance.
It is, therefore, an object of the invention to provide a stabilized power supply unit having a current limiting function, the power supply unit provided with an output transistor and a detection transistor impressed with the same control voltage as the output transistor, making the detection transistor outputting a detection current which is exactly proportional to the output current, thereby enabling accurate detection of the output current.
It is another object of the invention to provide a stabilized power supply unit having a current limiting function, adapted to detect the detection current only if it is necessary, thereby reducing power consumption of the unit.
In accordance with one aspect of the invention, there is provided a stabilized power supply unit for supplying an output voltage, comprising:
a voltage control circuit for outputting a voltage control signal in accordance with the difference between the output feedback voltage associated with the output voltage and a reference voltage;
an output circuit for outputting an output voltage under the control of the voltage control signal; and
a current limiting circuit having
a current detection unit for passing therethrough a detection current associated with the output current of an output circuit, under the control of the voltage control signal; and
a current limiting signal generation unit for generating a current limiting signal to limit the output current when the detection current exceeds a predetermined level, wherein
said current limiting circuit is provided with a voltage correction unit connected between the current detection unit and the current limiting signal generation unit, and supplied with the output voltage, and wherein
the voltage at the output end of the current detection unit set to, or close to, the output voltage.
The invention is provided with a current source that is enabled by a current-source control signal generated when the output current exceeds a predetermined level which is slightly below the maximum allowable limit of the output current.
In accordance with the invention, thanks to the voltage correction unit, the output voltage of the current detection unit is maintained at the same level as the output voltage at all times. Thus, a detection current exactly proportional to the output current can be obtained, irrespective of magnitudes of the output voltage and the output current. Accordingly, accurate current limitation of the output current can be carried out, limiting it exactly to the targeted over-current protective level.
Further, in accordance with the invention, since the source current is automatically turned off when over-current limitation is unnecessary (that is, when the output current is well below the allowable limit), the power consumption by the stabilized power supply unit can be suppressed. Moreover, whenever limitation of an over-current is required, the current source is securely turned on to enable the voltage correction unit, thereby carrying out an expected over-current limitation.